Quadrupole mass filters (QMFs) have become valuable analytical resources for generating mass spectra. QMFs are usually not used to analyze high m/z ions, due to the low frequency resonant circuits that drive them. Efforts to develop QMFs have been focused on increasing the maximum m/z value that the instruments are useful in detecting while maintaining a useful resolving power.
Several factors may limit the maximum m/z value that a QMF may be useful for resolving. Quadrupole mass filters are generally scanned by scanning RF amplitude with a fixed frequency. RF amplitude is proportional to the center of the m/z band transmitted by a QMF. Therefore, increasing RF amplitude increases m/z values that may be used with a QMF. The maximum m/z value that can be transmitted is limited by the maximum RF amplitude that can be achieved before electrical breakdown and/or power consumption become concerns. Also, for amplitude scans, the transmission efficiency for ions with the same energy and charge decreases as the m/z increases.
The center of the m/z band transmitted by a QMF is also proportional to 1/f2 where f is frequency. As frequency is decreased, the m/z value that may pass through a QMF increases. However, if frequency is not well-defined, resolving power may suffer. Although the ability to scan frequency over a wide range may also be desirable, known commercial frequency synthesizers are generally unable to provide accuracy over a range desirable to use QMFs to detect large m/z values.